A radar which is to be used to distinguish moving targets from fixed targets, such as, for example, objects on the ground, is often designed as a so-called pulse Doppler radar. This type of radar transmits radar pulses with a certain time interval between the pulses, the so-called pulse repetition interval, PRI. This interval can also, of course, be expressed as corresponding frequency, the so-called pulse repetition frequency, PRF.
Where a pulse Doppler radar is used on mobile platforms which are able to move at high speeds, it is often necessary to choose such a high PRF that an unambiguous range to the detected target cannot be obtained. PRF's which are so high that an unambiguous range cannot be obtained are often called MPD or HPD (Medium/High Pulse Doppler).
One of the difficulties of obtaining an unambiguous target range in the use of MPD or HPD is due to the fact that detected targets give rise to a plurality of detections, so-called primary detections, during the period in which the targets are illuminated by the so-called main beam of the radar antenna, a period which is also known as the dwell time. These primary detections will appear in many of the range resolution elements of the radar, the so-called range gates. In order to obtain the correct target range, it is necessary, in other words, to know which of the range gates in which the detection is found that corresponds to the actual range of the target from the radar.
A conventional method for obtaining an unambiguous target range in the use of MPD or HPD is to use more than one PRF during the period in which a target is illuminated by the main beam of the radar. In order to obtain an unambiguous target range from a detection, the information from the range gates of the various PRF's is compiled and compared with set criteria. These criteria are often of the M/N type, in which N can be the total number of PRF's used, and M is an integer less than and distinct from N.
A drawback with known methods for the use of criteria of the M/N type is that so-called "false targets" or "range ghosts" may be obtained, in other words indications of targets which are not there. These range ghosts are most often due to a mix of primary detections from a plurality of targets which are perceived by the radar, on account of the methods used, as belonging to one and the same target.
U.S. Pat. No. 4,584,579 sets out a method for reducing the number of range ghosts in a pulse Doppler radar. In brief, the method can be said to constitute an iterative process employing two different, non-variable criteria. According to the process, knowledge of detection history is also used, as well as separate processes for removing different types of false targets. A drawback of this method can thus be said to be that is calls for relatively highly complex equipment. Furthermore, the method does not reduce the volume of range ghosts to the extent which is desirable.